I dont think MOSS with pacman frogs is a good idea, just the coconut fiber will do, from what i have read they can ingest MOSS and become impacted. As far as changing substrate completely(Coconut fiber) i have read somewhere like every 3 weeks or so will be fine. Also remember NOT to spray them or give them water directly from the sink, it contains too many chemicals where the frog will ingest it thru its skin, have water available that has sat out for a 24 hour period and use that for water changes and spraying. I use bottled water but that can get expensive. Sink water has lots of chlorine, but it will evaporate with in 24 hours when left sitting out, then it will be safe for frog use.